1. Field of the Invention
The present invention relates to a nonreciprocal circuit device and, in particular, to a nonreciprocal circuit device, such as an isolator or a circulator, used in microwave bands.
2. Description of the Related Art
A nonreciprocal circuit device, such as an isolator or a circulator, has known characteristics that allow for transmission of a signal in a predetermined direction and not in a reverse direction. Because of these characteristics, for example, the isolator is used in a transmitter circuit of a mobile communication device, such as an automobile telephone or a cellular phone, for example.
Generally, this type of nonreciprocal circuit device includes a magnet assembly composed of ferrite provided with a center electrode and a permanent magnet for applying a direct current magnetic field thereto and a predetermined matching circuit element composed of a resistor and a capacitor.
International Publication No. 2006/080172 describes a 2-port isolator in which a coupling capacitor is connected between an input port and an output port for making insertion loss low. Japanese Unexamined Patent Application Publication No. 2006-211373 describes a 2-port isolator in which a coupling inductor is connected between an input port and an output port for the same purpose. Although it is possible to obtain preferable insertion loss with these isolators, attenuation of unnecessary waves, such as second and third harmonic waves, is not considered or addressed in the prior art isolators.